This invention relates to motion-picture projectors and more particularly to dual-sprocket rotation for conveying film through a projector at high speeds with proportionately long-enduring stops intermittently at film gates without flutter, wear or noise of film in intermittent motion and without damage or danger from malfunction of the projection system.
Currently, film projectors employ various types of film looping between input and take-up reels when film is being conveyed intermittently through film gates. Film gates are known also as light gates but will be referred to as film gates for consistency in this document. Film loops in conventional systems compensate for rapid changes of un-reeled length of film above and below a film gate as a result of timed stops of the film at the film gates between evenly synchronized input and take-up of the film by the respective reels. Traditional film projection systems have only a bottom film-transport wheel, called a bottom intermittent sprocket, which pulls the film through a film gate. Unfortunately, such traditional systems require high friction at the film gates to prevent the film from over shooting the film trap.
A variety of prior-art means and methods have been employed to control but not to eliminate film looping between top and bottom sprockets while also eliminating detrimental flutter, wear, noise, damage and danger from operation of a projection machine as taught by this invention.
Examples of different but related projection systems and methods are described in the following patent documents. U.S. Pat. No. 3,303,981, issued to Wiese, described top and bottom film sprockets that were rotated simultaneously in order to drive film through a film gate in the most nearly similar manner in relation to some aspects of this invention that is known. It was limited, however, to a fixed frame 11 that functioned in relation to a moveable frame 12 with a complex indexing means that generated and controlled a relatively small loop that was, nevertheless, a "film-gate loop" between top and bottom film sprockets 13 and 14. Instead of eliminating the film-gate loop with better means and methods, it attempted to minimize the problem in ways that perpetuated its basically detrimental features and taught against a thorough solution that is made possible by this invention.
Two U.S. Pat. Nos. 4,022,525 and 4,253,749, issued to Boudouris, also perpetuated means and methods for forming a film-gate loop between top and bottom film-conveyance sprockets. The '749 Boudouris patent provided for forming a looped film at a distance from a surface of a film gate. This "distance looping" was a prior-art continuation of the film-loop problem that apparently was recognized and attempted to be solved by an indexing system that was similar in principle to the above Wiese patent. The '525 Boudouris patent had larger film-loop problems that were diminished but not eliminated by the latter '749 Boudouris patent.
Other known projection systems are further yet different but pertinent to understanding the significance of this invention. U.S. Pat. No. 4,537,482, issued to Weiner, taught a barrel cam for intermittent transfer of film-gate sections of film between top and bottom film loops. German Application Number DE 3324-911-A, by Thiele, taught a controlled-motorization means between top and bottom film loops. U.S. Pat. No. 4,150,886, issued to Merkel, et al., taught a double-sided gear-belt drive of film-gate sections of film between film loops that were regulated loosely by top and bottom capstan gearing. U.S. Pat. No. 3,937,378, issued to Kopernicky, described detent control of film loop. U.S. Pat. No. 3,494,524, issued to Jones, taught a circumferential loop-control mechanism.
Contrary to the prior art, the present invention provides precision film placement in the film gate by using two synchronized film sprockets that accelerate and decelerate simultaneously. Thus, each frame of film is accurately placed in the same position as the previous frame.
On the other hand, prior art projection systems use pressure friction and registration pins or loops through the film gate assembly to perform this task. Such friction must be overcome to pull the next frame into position. Although the latter friction use may work satisfactorily with 35 millimeter film, it does not work well with larger film stocks and frame sizes, as the friction which is used and must be overcome increases logarithmically. The increased film mass and surface friction requires large film movements and results in increased mechanical failure and film wear.
The present invention eliminates the need to use friction by having a deceleration sprocket before the film gate which also eliminates any slack in the film so the film cannot flutter or jump.